1. Field of the Invention
The invention relates to a piston-cylinder assembly including a cylinder and a piston rod which is axially displaceably arranged in the cylinder, the piston rod adopting a stroke position determined by positioning equipment when no external forces are acting on the piston-cylinder assembly.
2. Description of the Related Art
There are applications in which a piston-cylinder assembly is required to adopt a particular stroke position without external force. An example of such an application is when the piston-cylinder assembly is connected to a switching lever which adopts a central switching position and is moved to a switching point by an actuating force in a first direction. When the actuating force is removed, the piston-cylinder assembly is to resume the central switching position. In the event of an opposed actuating force in a second direction, the piston-cylinder assembly is also required to resume the original central switching position after actuation of the switching lever. Of course, the piston-cylinder assembly may be required to aim for a position other than the central switching position after actuation, as defined by a specific application.
For such applications, a piston-cylinder assembly is disclosed in reference WO 98/40232 as a bidirectionally acting damper with a self-centering mechanism. The principal structural groups of this damper include a cylinder and a piston rod which is axially displaceably arranged relative to the cylinder with a part-length of the piston rod within the cylinder. The piston rod adopts a stroke position which is determined by positioning equipment when no external displacement forces act on the assembly. The positioning equipment includes an energy accumulator which acts in the inward and outward movement directions of the piston rod as a function of the stroke position of the piston rod.
In this damper, an assembly problem exists in that a circumferential bead is required to be produced as a final step of the method on a transmission pipe as part of the positioning equipment. The bead serves as a stop for the entrainment of a sliding sleeve. The production of the bead introduces a the manufacturing risk that the shape of the transmission pipe will be adversely influenced here which may cause functional problems of the piston-cylinder assembly. In addition, inaccuracies regarding the intended centered stroke position may be expected.
A further disadvantage of this damper in comparison with a conventional damper is that the transmission pipe introduces a substantial additional weight. The transmission pipe has to transmit tensile and compressive forces and is dimensioned accordingly. Furthermore, the transmission pipe has a length approximately corresponding to the sum of the length of the cylinder and half the inward travel of the piston rod. Such a long and rigid pipe is necessarily heavy.
Furthermore, both the inner diameter and the outer diameter of the two sliding sleeves have to be produced to very narrow tolerances in order to achieve sufficiently accurate guidance. If the production tolerances selected are too wide, jamming forces or skewed positions of the transmission pipe may arise. However, very narrow tolerances seriously increase costs.
It is an object of the present invention to produce a piston-cylinder assembly having two principal structural groups including a cylinder and a piston rod which adjust automatically to a default stroke position and can be produced at a reduced production cost by comparison with the prior art.
The object of the present invention is met by a piston-cylinder assembly having a tension spring arrangement and a compression spring arrangement used as energy accumulators. Each of the compression spring arrangement and the tension spring arrangement have a first end that acts in a stationary manner on one of the two principal structural groups and a second end that is operatively connected to the other principal group when the piston rod is outside the default stroke position.
The present invention does not require a transmission pipe and thereby obviates that problems of the prior art associated therewith.
Accordingly, the number of parts has been reduced compared to the prior art and the use of a tension spring arrangement and a compression spring arrangement allows different restoring forces to be achieved for different piston rod positions.
The compression spring arrangement may be arranged outside the cylinder such that its second end is operatively connected via a switching device supporting the forces of the compression and tension spring arrangements with one of the two principal structural groups. The switching device effects the separation of the effect of the force on the piston-cylinder assembly as a function of the piston rod position, the compression spring arrangement not being active in the area of action of the tension spring arrangement.
To this end, the switching device has a sliding sleeve which is axially movably arranged relative to the two principal groups. The sliding sleeve blocks the effect of the force of one of the two spring arrangements via a stop from a default stroke position of the piston rod.
The two spring arrangements may engage on the same sliding sleeve. Ultimately, there is only one moving element, in comparison with the three moving elements known from the above-mentioned prior art.
To simplify the structural arrangement, the sliding sleeve is mounted to be axially movable on the cylinder. The sliding sleeve has a fit connection only in the area of contact with the cylinder. Double fits are not required because a transmission pipe is not required.
To achieve reliable restoration of the piston rod into the default stroke position, the compression spring arrangement is always under a prestress, independently of the stroke position of the piston rod.
The stop is formed on the piston rod outside the envisaged stroke range of a piston arranged on the piston rod so that the fixing of the stop to the cylinder does not exert any influence on the track properties of the cylinder for the piston.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.